1. Field of the Invention
The present invention relates to a separator-equipped air electrode for a metal-air battery.
2. Description of the Related Art
One candidate of innovative batteries is a metal-air battery. In a metal-air battery, which is fed with oxygen (i.e., a positive-electrode active material) from air, the space in a battery container can be maximally filled with a negative-electrode active material. Thus, the metal-air battery can achieve high energy density in principle.
For example, a zinc-air battery, which contains zinc as a negative-electrode active material, includes an alkaline electrolytic solution (i.e., an aqueous solution of an alkali, such as potassium hydroxide) and a separator for preventing the short circuit between the positive and negative electrodes. During a discharge mode of the battery, O2 is reduced to generate OH− at the air electrode (positive electrode) and zinc is oxidized to generate ZnO at the negative electrode as illustrated in the following formulae:O2+2H2O+4e−→4OH−  Positive electrode:2Zn+4OH−→2ZnO+2H2O+4e−  Negative electrode:
Several attempts have been made to use a zinc-air battery as a secondary battery. Unfortunately, the battery poses a problem in that the reduction of generated Zn(OH)42− (i.e., an ionic species soluble in the electrolytic solution) forms zinc dendritic crystals (i.e., dendrites) during a charge mode, and the dendrites penetrate the separator to cause the short circuit between the negative electrode and the positive electrode. The battery also poses a problem in that carbon dioxide contained in air permeates the air electrode and dissolves in the electrolytic solution, and the resultant carbonate ions cause the degradation of the electrolytic solution. In order to solve such problems, Patent Document 1 (WO2013/073292) discloses a zinc-air secondary battery including a separator composed of a hydroxide-ion-conductive inorganic solid electrolyte, wherein the inorganic solid electrolyte is disposed on one surface of an air electrode, and the inorganic solid electrolyte is a hydrothermally solidified dense layered double hydroxide (LDH) represented by the formula: M2+1-xM3+x(OH)2An−x/n.mH2O (where M2+ represents a divalent cation, M3+ represents a trivalent cation, and An− represents an n-valent anion). The air electrode, which is disposed on the inorganic solid electrolyte (separator), is composed of a particulate catalyst, such as platinum, and a particulate electrically conductive material, such as carbon material.
The aforementioned problems may occur in a lithium-air secondary battery. In connection therewith, Patent Document 2 (WO2013/161516) discloses a lithium-air secondary battery including an anion exchanger composed of a hydroxide-ion-conductive inorganic solid electrolyte, wherein the inorganic solid electrolyte is disposed on one surface of an air electrode, and the inorganic solid electrolyte is a hydrothermally solidified dense layered double hydroxide (LDH). The air electrode, which is disposed on the inorganic solid electrolyte (anion exchanger), is composed of a particulate catalyst, such as platinum, and a particulate electrically conductive material, such as carbon material.
Various air electrodes have been proposed for improving the performances of metal-air batteries. For example, Patent Document 3 (JP5207407B) discloses an air electrode having a layered structure including a polymeric anion-exchange membrane and an air electrode catalyst layer containing an anion-exchange resin, and describes that the air electrode maintains high hydroxide ion conductivity. Patent Document 4 (JP2013-201056A) discloses an air electrode catalyst layer containing a layered double hydroxide and a transition metal catalyst, and describes that an anion exchanger may be disposed on an electrolyte-side surface of the catalyst layer, and the anion exchanger may be an anion-conductive electrolyte membrane containing a layered double hydroxide (in particular, a coating membrane prepared with a dispersion). Patent Document 5 (JP5158150B) discloses an air electrode for a metal-air battery, the air electrode containing an air electrode catalyst and an electrically conductive material, wherein the air electrode catalyst contains a layered double hydroxide. Patent Document 6 (JP5221626B) discloses an air electrode for a metal-air secondary battery, the air electrode containing an air electrode catalyst, an air electrode electrolyte, and an electrically conductive material, wherein the air electrode electrolyte contains a layered double hydroxide, and the amount of the layered double hydroxide is 10 to 20 mass % relative to the total mass (taken as 100 mass %) of the air electrode catalyst and the layered double hydroxide. Each of the air electrodes disclosed in Patent Documents 3 to 6, however, does not include a separator composed of a dense ceramic material, such as a hydrothermally solidified layered double hydroxide (LDH) disclosed in Patent Document 1.